PROMs conventionally comprise a plurality of lateral or vertical fuses wherein a predetermined number of fuses are located in a row coupled to a word line and another predetermined number of fuses are located in a column coupled to a bit line. In other words, each fuse is uniquely coupled in a combination of columns and rows.
When a fuse is intact (electrically closed), it represents one digital state, i.e., "1". When the fuse is programmed or "blown" (electrically open), it represents a different digital state, i.e., "0". PROMs are manufactured with the fuses in the closed state and are subsequently programmed by applying a relatively high voltage between selected word lines and bit lines to open specific fuses.
Vertical fuses typically comprise two back to back diodes having common anodes. The resistance of vertical fuses, and therefore their programmability, may be predicted fairly easily. Lateral fuses typically comprise a film of a material, i.e., nichrome, horizontally formed across a surface of the semiconductor chip. The resistance of lateral fuses can vary substantially; therefore, their programmability is not as predictable.
One previously known method of testing the programmability of lateral fuses was to have test rows and test columns, in addition to the normal rows and columns, manufactured on the PROM; program fuses between the test rows and test columns; and then test for fuses actually programmed. This would give some indication of the probability of the remaining fuses on the ROM of being programmed sucessfully.
However, this method required an abundance of additional space on the chip, consumed a large amount of testing time, and the results from the testing of the test fuses did not always correlate with the programming of the normal fuses.
Another previously known method of testing the programmability of lateral fuses was to program a sample of a group of ROMs; test the programmed sample for fuses actually programmed; and predict the programmability of the remaining ROMs from the group based on statistical probabilities.
However, this method destructively consumed the number of PROMs used in the sample and the results from the testing of the sample did not always correlate with the actual programming of the remaining group.
Thus, what is needed is an on-chip circuit that allows for the verification of the programmability of lateral fuses.